Various motors are widely used in many different applications. With the development of power electronics, brushless direct current (BLDC) motors are being used in more and more applications especially in the fields of office automation, home appliances, automotive accessories, etc., for its simple structure, reliable electromagnetic performance, and low maintenance requirements.
Currently, many BLDC motors are slotted motors, and the stator of slotted motors usually comprises many teeth and slots of regular distribution, located in radial direction along its internal circumference, with the teeth and slots extending along the whole axial length of the stator. Using various techniques well known by those skilled in the art, the stator winding is embedded into the slots according to a certain phase order. However, this kind of slotted motor has the disadvantages of slot effect, magnetic hysteresis loss, eddy current loss, magnetic saturation, being inconvenient to wind, etc.
Slotless motors were developed as an alternative to slotted motors, as described for example, in U.S. Pat. No. 5,197,180. In a slotless motor the winding, which may be the stator or the rotor, is formed by winding a single core insulated wire (referred to as magnet wire) about a frame to form a hollow coil having multiple turns. A number of connection tappings may be formed between groups of the turns to form the phase windings. The frame is removed and the coil is pressed or flattened to form a double layer web of magnet wire which is then rolled end to end to form a cylindrical structure. Although this slotless motor can solve some of the disadvantages of the slotted motors, it does have a disadvantage of high eddy current loss within the stator winding under high current, high speed operating conditions. This is due to the structure of the slotless motor.
FIG. 1 is a cross-sectional schematic view of a typical BLDC slotless motor. The motor has a stator core 10, a stator winding 20 formed by a coil of magnet wire 22 fixed to the stator core and a permanent magnet rotor 14. Lines 11 represent the magnet flux path though the stator and rotor. As shown in FIG. 1, the coil of the slotless motor is a part of the main magnetic circuit. The eddy current loss and the magnetic flux density produced in the coil, the alternating frequency of the magnetic flux and the diameter of the wire meet the following relationship: P∝(Bfd)2, wherein, P represents the eddy current loss produced in the coil, B represents the magnetic flux density of the magnetic field passing through the coil, f is the alternating frequency of the magnetic flux, and d represents the diameter of the magnet wire of the coil. So, when the slotless motor is used with high current, high rotating speed, and with the magnetic flux density of the magnetic field passing through the coil being comparatively large and the magnet wire being thick, the alternating frequency of the magnetic flux is high, producing comparatively large eddy current losses in the coil.